1. Field of the Invention
The present invention relates to a data processor and a data processing method. More particularly, the present invention relates to a data processing method and a data processor, characterized by a relationship between a dot arrangement pattern which forms an image, and a mask pattern for printing the image by dividing the pattern into several numbers of times.
2. Description of the Related Art
Along with the recent proliferation of information processing devices such as personal computers, a printing apparatus as an image formation terminal has also been rapidly developed and widely used. Particularly, among various printing apparatuses, an ink jet printing apparatus, which performs printing on various printing media by ejecting ink as droplets, can carry out a high-density and high-speed printing operation with low noise. Moreover, color printing can be easily handled with the ink jet printing apparatus, and the device is inexpensive. As described above, since the ink jet printing apparatus has many excellent features, the apparatus has now become a mainstream of printing apparatuses for personal use.
Advancement in an ink jet printing technology facilitates higher-quality printing at higher speed and lower costs, and significantly contributes to the effect of making the printing apparatuses popular among personal users, in combination with widely-used personal computers, digital cameras and the like. However, due to such popularity of the printing apparatuses, the personal users desire for further improvement in image quality produced with the printing apparatuses. Particularly, demanded recently are a print system which allows easy printing of photographs at home, and image quality equivalent to that of silver-halide photography.
However, due to its manufacturing processes, slight variations inevitably occur, among a plurality of nozzles in an ink jet printing head, in a direction of ejecting ink and in the amount of ink. Moreover, in a serial type printing apparatus, the amount of sub-scanning (paper feeding) performed between each of printing scans includes some structural errors. Such errors and variations cause adverse effects on images, such as stripes and density unevenness, on a printing medium onto which the ink is applied.
In order to avoid such adverse effects on images, a printing method known as multi-pass printing is often adopted in a serial type ink jet printing apparatus.
FIG. 1 is a schematic view of a printing head and a printing pattern for explaining the multi-pass printing. Reference numeral 1001 denotes a printing head. Here, for simplicity, the printing head is supposed to have 16 nozzles. As shown in FIG. 1, the nozzles are divided into four groups, first to fourth, and each of the groups includes four nozzles. Reference numeral 1002 denotes a mask pattern in which areas that can be printed with the respective nozzles (print permission pixels) are shown in black. Patterns to be printed with the respective nozzle groups are in a completing relationship with each other. These patterns are superimposed on one another to complete printing in a region corresponding to 4×4 areas.
Patterns denoted respectively by reference numerals 1003 to 1006 show how an image is completed by repeating printing scans. Every time each of the printing scans is completed, a printing medium is carried by a width of each nozzle group in a direction indicated by the arrow in FIG. 1. Accordingly, in the same region (a region corresponding to the width of each nozzle group) of the printing medium, the image is completed with four printing scans.
By adopting the multi-pass printing as described above, the adverse effects on images, such as aforementioned stripes and density unevenness, can be reduced. This is because, even if there is a variation in ejection characteristics of the respective nozzles or in a transfer amount, these characteristics are widely dispersed to make the variation less noticeable.
By using FIG. 1, the description has been given by taking, as an example, four-pass printing in which four printing scans are performed for the same image region. However, the multi-pass printing is not limited to the above case. It is also possible to adopt two-pass printing for completing an image with two printing scans, or to adopt a configuration for completing an image with five or more printing scans. The larger the number of passes is, the more widely the variation in the ejection characteristics of the respective nozzles or in the transfer amount is dispersed. Thus, a smoother image can be obtained.
In order for the multi-pass printing to sufficiently achieve the foregoing effects, predetermined conditions are required between a dot arrangement pattern and mask pattern of an image, particularly, in a halftone.
FIG. 2 is a view for explaining the foregoing conditions. In FIG. 2, reference numeral 2001 denotes dot data given to a region of 4 areas×8 areas. In this dot data, black areas show areas where dots are printed, and white areas show areas where dots are not printed. Reference numerals 2002 and 2003 denote two kinds of mask patterns in a completing relationship with each other. The two kinds of mask patterns are applied to the image region described above. Here, reference numeral 2002 denotes the mask pattern to be used for a first printing scan, and reference numeral 2003 denotes the mask pattern to be used for a second printing scan. In the mask patterns, a black area indicates an area in which a dot is allowed to be printed in a printing scan (hereinafter also referred to as a “print permission area”), and a white area indicates an area in which a dot is not allowed to be printed (hereinafter also referred to as a “print non-permission area”). The areas in which printing is actually performed in each of the printing scans are obtained by a logical product (AND operation) of the dot data 2001 and the mask pattern 2002 or the mask pattern 2003. Reference numerals 2004 and 2005 show the results thereof. Here, the areas in which the printing is actually performed in each of the printing scans are shown in black, and the areas in which no printing is performed are shown in white. As is clear from FIG. 2, in this example, a significant difference is caused in the number of areas, in which the printing is actually performed, between the first printing scan and the second printing scan. Specifically, ejection characteristics of nozzles used in the first printing scan have a significant influence on an image. As a result, it is hard to achieve the effects of the multi-pass printing.
For the above reason, in order to sufficiently achieve the effects of the multi-pass printing, it is necessary to print approximately the same number of dots in each of a plurality of printing scans performed for the same image region. This is because, if the number of dots to be printed drastically varies from scan to scan, the variation in the ejection characteristics of the respective nozzles or in the transfer amount is not dispersed. For this reason, adverse effects on images, such as stripes and density unevenness, are not reduced.
Here, the description has been given by using the pattern 2001 as an example. Meanwhile, dot data to be printed undergoes diverse changes according to a gradation value and a pulse-surface-area modulation (a quantization method) to be adopted. In consideration of such circumstances, there has already been disclosed a technology of preparing a mask pattern not synchronous with the pulse-surface-area modulation adopted in the multi-pass printing (see Japanese Patent Laid-Open No. H5-31922).
There has also been disclosed a technology and a method for generating a mask pattern in which print permission areas and print non-permission areas are randomly arranged, as a mask pattern which satisfies the foregoing conditions as much as possible regardless of inputted image data with any gradation by use of any pulse-surface-area modulation (see, for example, Japanese Patent Laid-Open No. H7-52390).
Furthermore, in the multi-pass printing, various mechanical problems unique to a printing apparatus can be prevented from appearing on images by further contriving arrangement of the mask pattern while giving consideration to the foregoing conditions.
For example, Japanese Patent Laid-Open No. 2002-144552 discloses a method for applying a mask pattern which has excellent dispersion properties and suppressed low-frequency components. In the multi-pass printing, if a printing position of one printing scan is shifted from other printing scan, a design (texture) of an employed mask pattern is made visible. Even in such a case, by adopting the method disclosed in Japanese Patent Laid-Open No. 2002-144552, a mask pattern itself, which has excellent dispersion properties, and of which appearance is favorable, is made less obtrusive, in other words, less visible. Thus, there is hardly any influence on image quality.
Meanwhile, as to what is termed as binarization processing for converting multi-level gradation data, which indicates density of an image to be printed, into dot data indicating whether or not each of ink droplets is printed on a printing medium, many methods have already been proposed and disclosed. Basically, any one of the methods can be adopted. However, in recent years when a printing resolution of a printing apparatus and the number of ink colors are being increased, it may be too heavy a burden to perform entire image processing of all colors at the same resolution as the printing resolution. For this reason, for example, the following printing system has been recently provided. Specifically, after a host device performs main image processing at a resolution lower than a printing resolution, quantization processing is performed for reducing the number of levels in the gradation value of each pixel to the several levels thereof. Thereafter, final binarization processing is further performed by a printing apparatus. In this case, each pixel outputted by the host device is expressed in gradation with multiple levels of density. Thus, the above system can be considered as being suitable for use that places importance on gradation properties such as photographic image quality.
As to a method for converting several stages of multi-level density data into binary data, some proposals and implementations have already been given. For example, Japanese Patent Laid-Open No. 1997-46522 discloses a method for expressing gradation by printing or not printing four dots within 2×2 areas for one input pixel having five stages of gradation values. Furthermore, the above patent document also discloses a method for preparing a plurality of dot arrangement patterns within 2×2 areas for the same gradation value, and then by sequentially or randomly arranging these dot arrangement patterns. By use of the above method, the dot arrangement pattern for each stage of gradation is not fixed. Thus, a pseudo contour appeared when pseudo halftone processing is performed, what is termed as a “sweeping phenomenon,” which appears on an edge of an image, and the like are reduced. Moreover, the above patent document describes that the above method has an effect of averaging use of a plurality of printing elements arranged in a printing head. As described above, the method for converting the data of the several stages of multi-level density that the low-resolution pixel has, into high-resolution binary data is an effective technology for an ink jet printing apparatus which prints minute dots at a high definition. Such a processing method will be hereinafter referred to as dot arrangement patterning processing in the present specification.
As described above, in the recent ink jet printing system, high-quality output images equivalent even to the photographic image quality are achieved by using a random mask pattern and a mask pattern having high dispersion properties, while utilizing the dot arrangement patterning processing.
However, the random mask pattern and the mask pattern having high dispersion properties, which have heretofore been generally used, are not created by giving due consideration to characteristics of output data from the dot arrangement patterning processing used in the same printing system. The output data from the dot arrangement patterning processing has several gradation levels. The gradation levels are expressed with combinations of areas where dots are printed and areas where dots are not printed, within m×n areas (one area is a region where one dot is printed). In contrast, in the conventional mask pattern, one area or a plurality of adjacent areas, which are unrelated to the m×n areas, are set as a unit. Then, only randomness and dispersion properties of each unit are taken into consideration. Here, m and n respectively indicate positive integers, and at least one of m and n is an integer of 2 or larger.
In such a case, it is confirmed that interference occurs between a dot arrangement pattern, in which areas for printing or not printing dots are regularly set within a relatively narrow range, and a mask pattern having irregularity within a wider range than that of dot arrangement pattern. To be more specific, when the multi-pass printing is performed, a situation occurs where in some pixels formed in m×n areas all dots are printed at once, and in other pixels all dots are printed by separately several times.
A known phenomenon is that, even in a case where the same number of dots of the same ink color are printed for expressing the same hue in two pixels, a difference occurs in color development and density of the two pixels when the number of scans required for printing (the time required for completing printing) varies between the two pixels. Specifically, in the conventional combination of the dot arrangement patterning processing and the mask patterns, the number of times that dots within each pixel are printed, and the timings at which the dots are printed are different among the pixels. For this reason, color development and density in each of the pixels is unstable.